Various heat treatment methods are known for heat-treatment of steel by induction heating. In one of the known heat treatment methods, an induction-heating coil is placed near a portion of a workpiece to heat the portion, then the workpiece is turned for heating the other portions, and thus the entire of the workpiece is induction-heated up to a prescribed temperature. For turning the workpiece, a driving power source like a motor is necessary for turning a workpiece-supporting member.